A few summers ago, inspired by another strange obsession, I finally left my sky-high apartment in Notting Hill for a new home here in this old Victorian tenement among the tightly clustered streets of the old part of town. From my windows now, it is only possible to see other buildings - in finally leaving my old place with its airy perspectives I descended into the heart of the city - where, directly under me, deep beneath the surface, flows the river Fleet, the greatest of London's lost tributaries to the Thames.

Along with the Tyburn, the Westbourne, the Effra, the Walbrook and the Neckinger, the Fleet now exists only in the conscious life of the city by vague references in local street names and in the occasional watery incident in some deep basement or subterranean structure. But you can't get rid of a river. Flowing down from the Vale of Health on Hampstead Heath, filling Hampstead ponds as it goes, descending through Kentish Town and past King's Cross, the Fleet still winds its way down to the Thames as it ever did - only now suppressed into a channel deep below the concrete.

Inexplicably haunted by peculiar dreams of this lost river, for months I bluffed, bullied and badgered Thames Water into finally allowing me to visit it. Accompanied by a gang of sewermen, equipped with full waterproof gear and gas mask, I watched as an innocuous looking manhole cover was raised in a pavement south of Holborn Viaduct. Descending a slender ladder and edging along a narrow ledge, down another ladder and along winding brick passageways, we made our way deeper and deeper into the darkness, accompanied by the ever-increasing sound of rushing water.

We made our way for what seemed an age around twists and turns and along ancient brick branching walkways. Suddenly, we emerged from the cramp of the tunnels into a vast chamber whose floor was covered in churning water and at whose far end were two vertical pairs of giant iron gates. This is the mouth of the Fleet that now only pushes the gates open into the Thames at times of storm.

It was a spooky, confusing place and even my companions seemed keen not to linger. We waded upstream in the warm darkness, listening out for rats and seeing with some wonder the haunches of ancient bridges still buried in the walls above the banks. Smaller tributaries emptied themselves into the stream from either side and occasionally, high above, a patch of daylight could be glimpsed. At the confluence of two channels, we dredged the bed of the water where I found a Victorian silver sixpence and an old, battered silver ring. Eventually, after further explorations we left the stream and climbed upwards again to emerge blinking into the relatively sweet air of Clerkenwell. I spent the next couple of hours in the back of the sewermens' van, drinking tea, smoking fags and listening to tall tales of the subterranean world below the city, invisible and unimagined to most of its inhabitants.

Was it a symbolic journey? Was it, in fact, real? Later, I sometimes wondered if perhaps I had dreamt it too. I still have no idea why it seemed so important and yet, afterwards, city seemed different, I was different, everything was different and a whole chain of events began that day that continues to this.

When I'm long gone, when this city and all its complications are long gone, the rivers will still be there, flowing freely once more, out into the land, out into each other and out into the sea, just as they always did………